1. Field of the Invention
This invention relates to a superconducting logic circuit comprising Josephson junctions and superconducting inductors. In particular it relates to a unidirectional single-flux-quantum logic circuit which can be operated with less than a three-phase clock.
2. Description of the Prior Art
Since Josephson junctions operate very quickly and consume only a little power, significant attention has been paid to them as possible elements for high-performance computers. In general, Josephson junctions can be classified into two types; namely, the tunnel type and the bridge type. So far, tunnel type Josephson junctions with hysteretic effect of current-voltage characteristics have been used in digital circuits, while assigning the zero-voltage state and the finite-voltage state of the junction to the binary logical values respectively. On the other hand, as compared with the case of the tunnel type, the junction capacitances of the bridge type Josephson junctions are negligible, so that bridge type Josephson junctions are expected to operate faster than the tunnel type. Since bridge type Josephson junctions have no hysteretic effect in the current-voltage characteristics, the magnetic flux quantum (to be referred to as "flux quantum" hereinafter) is used as the information carrier in the logic circuits made of bridge type junctions.
Two groups of flux quantum logic circuits using the magnetic quantum as the information carrier have been proposed heretofore; namely, a group using one-junction superconducting quantum interference devices (to be referred to as SQUID's hereinafter) and another group using two-junction SQUID's. The group using the two-junction SQUID's has a shortcoming in that it is easily susceptible to the so-called latch-up or permanent transition of its Josephson junctions to finite-voltage state, so that its regular operating region is comparatively small.
In either group of the flux quantum logic circuits of the prior art, a polyphase clock having three or more phases is necessary to ensure unidirectional transmission of signal therethrough. With the polyphase clock, complicated polyphase wiring is indispensable, so that the process of producing such logic circuit is also complicated. Miniaturization of such a logic circuit with polyphase clock wiring into an integrated circuit (IC) has been attempted, but the clock wiring has been found to occupy an unduly large area in the IC. Besides, the transmission time per one step of the logic circuit depends on the period of the clock, so that the very high speed characteristics of the Josephson junctions cannot be fully utilized when a polyphase clock is used.